The present invention relates generally to methods and systems for processing digital imagery from a variety of optical imagery systems, and more particularly, to a method and system for producing digital orthophotos using sparse stereo photogrammetric configurations with or without externally DEM""s and new photogrammetric software tools.
In traditional digital orthophoto processes, digital imagery data typically are acquired by scanning a series of frames of aerial photographs which provide coverage of a geographically extended project area. Alternatively, the digital imagery data can be derived from satellite data and other sources. Then, the image data are processed on a frame by frame basis for each picture element, or pixel, using rigorous photogrammetric equations on a computer. Locations on the ground with known coordinates or direct measurement of camera position are used to establish a coordinate reference frame in which the calculations are performed.
During conventional orthophoto production processes, a DEM, or digital elevation model (DEM), is derived from the same digital imagery used in subsequent orthorectification, and this DEM has to be stored in one and the same computer file.
Then, the imagery data for each frame is orthorectified using elevation data obtained from the DEM to remove image displacements caused by the topography (xe2x80x9crelief displacementsxe2x80x9d). For many conventional processes, the steps of measurement are performed with the imagery data for each frame or for a pair of two frames having a 60% forward overlap. In traditional image processing systems, the measurement process is carried out primarily on the digital imagery accessed in pairs of overlapping frames known as a xe2x80x9cstereomodelxe2x80x9d. Subsequent photogrammetric calculations often are carried out on the digital imagery on a stereomodel basis. Orthorectification is carried out on the digital imagery on a frame by frame basis. These processes are time consuming and costly. For example, using traditional methods with high process overhead and logistical complexity, it can take days to process a custom digital orthophoto once the imagery has been collected.
After orthorectification of the individual frames, the orthorectified images are combined into a single composite image during a mosaicking step.
The disadvantages and limitations of the background art discussed above are overcome by the present invention which provides a method for producing a digital orthophoto from a block of overlapping images of a project area, including acquiring imagery data for the project area and performing all geometric coordinate transformations are performed prior to performing the image intensity interpolation operation. Thus, only one image intensity interpolation operation is performed, using the geometric coordinate data. This one-step resampling process avoids the situation in which multiple image resampling operations unnecessarily degrade the color content of the resulting imagery. In one embodiment, digital elevation model data is derived from the imagery data and an orthorectification operation is performed using the derived digital elevation model data. Then, the geoimage is transformed to a map coordinate system.
In accordance with another aspect of the present invention, orthophotos can be created in a variety of map coordinate systems without the need to convert or recompute DEM or photogrammetric solution data. Alternatively, an identity transformation can be used so that map coordinates and world coordinates are identical. With this process, given DEM data and photogrammetric solution data in a particular coordinate system, the orthophoto image data can be produced in any map coordinate system.
In accordance with a further aspect of the invention, there is provided a method for producing a digital orthophoto from a block of overlapping images for a project area. The method comprises acquiring digital imagery data representing the block of overlapping images of the project area and obtaining measurement data for tie and control points. A network constraint is introduced to a block bundle adjustment that assumes an average vertical direction for the imagery to support the process of self-rectification.
Further in accordance with the invention, there is provided a method for producing a digital orthophoto from a block of overlapping images for a project area. The method comprises acquiring digital imagery data representing the block of overlapping images of the project area; obtaining measurement data for a plurality of tie and a plurality of control points; performing photogrammetric interpolation adjustment using a block bundle adjustment process to obtain an elevation interpolation model including three-dimension ground points; fitting the elevation interpolation model between ground points; and interpolating digital elevation model elevation at arbitrary ones of the three-dimension ground points to support rectification in world coordinates.
Further in accordance with the invention, a method for improving the quality of tie points and control points measured in producing a digital orthophoto from a set of overlapping images of a project area comprises measuring a plurality of points within a block of images of the project area; performing a two-dimension, free network adjustment using the points measured within the block of images to simultaneously compute a two-dimension orientation transformation for each image in the block of images; performing a three-dimension, free-network adjustment using the results of the two-dimension, free-network adjustment; assessing the quality of the points for the block of images using the results of the three-dimension, free-network adjustment; refining the measurement of at least one of the points; and performing at least a further three-dimension, free-network adjustment.
Further in accordance with the invention, there is provided a method for measuring tie points and control points in producing a digital orthophoto from a block of overlapping images of a project area. The method comprises the steps of displaying the overlapping images in pairs, and measuring corner points in each image pair to obtain point data. Then, a two-dimension, free network adjustment is performed, using the point data obtained by measuring points within the block of images to simultaneously compute a two-dimension orientation transformation for each image in the block. A plurality of additional tie points are measured within the block of images of the project area. The two-dimension, free-network adjustment is performed prior to measuring the additional points.
The invention further provides a method for providing digital elevation model data for use in producing a digital orthophoto of a project area. The method comprises acquiring digital elevation model data from at least first and second sources; prioritizing the digital elevation model data acquired from the first and second sources; storing the digital elevation model data for at least the first and second digital elevation models in a memory in a predetermined order to produce a digital elevation multimodel. The digital elevation model data having the highest priority is accessed to provide elevation data for use in performing a first geometric transformation. Then the digital elevation model data having the next priority is accessed to provide elevation data for use in performing a second geometric transformation.
A method for producing a digital orthophoto for a project area, the method comprising the steps of using an uncalibrated camera to obtain a series of aerial photographs of the project area for providing digital imagery representing a block of overlapping images of the project area; introducing at least one perspective camera model parameter into a bundle block adjustment program as an unknown parameter pertaining to the taking camera; performing a bundle block adjustment operation using the digital imagery data to calculate the unknown parameter; supplying the calibrated parameter data and bundle adjustment result data to an orthorectification process; and performing an orthorectification operation using the calibrated parameter data and the bundle adjustment data.
The invention further provides a method for creating orthophoto images from imagery without stereo overlap and without availability of an external digital elevation model. The method comprises performing a photogrammetric adjustment using tie points and control points for the sparse stereo geometry to provide a three dimensional coordinate value for a plurality of ground points, the ground points corresponding to all points which have been measured in two or more images, and the ground points form a sparse sampling of the ground digital elevation model with each ground point having a known elevation value expressed in world space and a planimetric location expressed in world space. A digital elevation model is interpolated between the ground points for use in producing a digital orthophoto for a project area.
With this invention, there is provided a method and system for producing digital orthophotos from sparse stereo configurations which consist of photographic block geometries exhibiting an overlap much smaller than 60%, together with a method and system which uses various DEM files allowing an automated selection based on geographic location.